[codeview] Fully initialize DataSym when mapping from YAML

[codeview] Fully initialize DataSym when mapping from YAML

In the object file, the section index and relative offset are typically
zero, so make these YAML fields optional with a default.

It looks like there may be more partially initialized symbol records,
but this should fix the msan bot.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305842 91177308-0d34-0410-b5e6-96231b3b80d8